1. Field of the Invention
This invention concerns a drying furnace for use in heating and drying paint films coated on articles and, particularly, it relates to a drying furnace for use ih coating drying in which ducts are disposed along the ceiling at the inlet and the exit zones of a tunnel-like furnace main body for preventing resinous substances from being formed therein.
2. Description of the Prior Art
In a drying furnace used for use in coating drying in which car bodies just after the coating applied thereto is baked and dried during transportation of the car bodies within a furnace main body of tunnel-like shape, noxious and smelly ingredients such as organic solvents, paint resins and curing agents are released from the coated films on the car bodies when they are heated to a high temperature within the furnace.
As the concentration of the noxious and smelly ingredients is increased, sticky resinous substances that may cause yellowing or ply-separation of coated films are yielded in a great amount at the inlet zone and the exit zone of the furnace main body where the internal temperature is lowered due to the intrusion of atmospheric air. These are deposited to the ceiling and fall in the form of liquid droplets to the surface of the car bodies thereby causing defective coatings and even resulting in fire danger. Countermeasures for reducing the formation and deposit of such resinous substances have been proposed, for instance, as disclosed in Japanese Patent Laid Open Nos 10194/1985 and 183070/1985. This prior art discloses providing heating means at the inlet and the exit of a furnace main body for keeping the temperature of the exhaust gases from lowering, thereby preventing the resinous substances from being yielded.
As shown in FIG. 3 (which shows a prior art coating-drying furnace having conventional resinous substance-abatement means), exhaust gases in the furnace main body 1 are discharged through an exhaust duct 2 to a deodorizing and purifying device 3 and, in addition, a hot gas at a high temperature is supplied under circulation to each of ducts 4 and 5 disposed along the ceiling of the inlet zone 1a and the exit zone 1b of the furnace main body to prevent the temperature of the exhaust gases from lowering in the inlet zone 1a and the exit zone 1b.
In a drying furnace for baking and drying the paint films coated on a car body B just after the electrodeposition of such coating films, hot gas recycling ducts 6 referred to as "far-infrared dust" are disposed along the right and left walls on the side of the inlet zone 1a in the furnace main body 1. A hot gas at a high temperature from 250.degree. to 300.degree. C. is supplied into the ducts 6 under circulation from hot gas generation device N.sub.1 so that the coated wet films on the car body B are dried to some extent by radiation heat at about 200.degree. C., radiated from radiation plates 7 disposed in front of the ducts. Then, after the surface of the coated films has been dried, the films are baked and dried by the hot gas at a temperature from 170.degree. to 180.degree. C. blown out from the blowing ports 9 of hot gas duct 8. In this way, for rapidly increasing the temperature of the car body B conveyed into the furnace main body 1 and drying the coated wet films just after the electrodeposition with good luster without depositing dust or the like, the surface of coated films is at first dried by radiation heat at about 200.degree. C., radiated from the radiation plates 7 of the hot gas recycling ducts 6. Then, a hot gas at about 170.degree. to 180.degree. C. blown out from the blowing ports 9 of the hot gas duct 8 and circulated under convection within the furnace is directly blown to cause a curing reaction in the coated films at a good heat efficiency.
Then, the hot gas at a high temperature supplied to the inside of the hot gas recycling ducts 6 is supplied for energy saving to the inside of the duct 4 disposed at the inlet 1a of the furnace main body 1 for preventing the formation of resinous substances and, thereafter, again heated to a temperature about from 250.degree. to 300.degree. C. while receiving heat from burner 10 in the hot gas generating device N.sub.1 and supplied under circulation to the inside of the hot gas recycling ducts 6 by recycling blower 11. Further, a portion of the hot gas sent to the inside of the hot gas duct 8 is supplied to the inside of the duct 5 disposed at the exit 1b of the furnace main body for preventing the formation of resinous substances and then heated to a temperature about from 170.degree. to 180.degree. C. while receiving heat from burner 13 in hot gas generating device N.sub.2 together with the air in the furnace sucked from suction blower 14 and then supplied under circulation by recycling blower 14 to the hot gas duct 8.
In this case, since the duct 4 disposed at the inlet 1a of the furnace main body 1 for preventing the formation of resinous substances is heated by a clean hot gas at high temperature (at 250.degree. to 300.degree. C.) under circulation to the hot gas recycling ducts 6, the temperature at the inside of the inlet 1a can be maintained at a high level to reliably prevent the formation of resinuous substances, as well as to keep the inside of the duct 4 per se free from contamination due to the deposition of resinous substances.
However, since the hot gas at a relatively low temperature about from 170.degree. to 180.degree. C. is supplied from the hot gas duct 8 to the inside of the duct 5 disposed at the exit 1b of the furnace main body 1 for preventing the formation of resinous substances, the temperature at the surface of the duct 5 is liable to be lowered under the effect of the atmospheric air at a low temperature that intrudes from the exit 1b to yield and deposit resinous substances onto the surface of the duct 5. Amounts of the deposited resinous substances fall dropwise, if accumulated to a certain extent, to the surface of the car body B and result in the serious drawback of a defective coating. Particularly, in other types of drying furnaces than the electrodeposition furnace described above, since the temperature of the hot gas supplied under circulation to the inside of the hot gas duct 8 is extremely low such as from 130.degree. to 140.degree. C. in a sealer furnace, from 150.degree. to 160.degree. C. in an intermediate coating furnace, from 120.degree. to 140.degree. C. in a water polishing furnace and from 140.degree. to 150.degree. C. in a top coating furnace, great amounts of resinous substances are formed and deposited due to the lowering in temperature.
Further, since a hot gas containing noxious and smelly ingredients circulated in the inside of the furnace and sucked from the suction duct 12 is supplied under circulation to the inside of the duct 5 for preventing the formation of resinous substances, there has also been a drawback that the resinous substances are formed and deposited on the inside of the duct 4 as well thereby necessitating extremely troublesome cleaning work.